An electrical or power substation can involve electricity transmission and distribution systems where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer or load, and the voltage may be transformed in several steps.
Industrial setups involve intense energy consumption and include one or multiple dedicated substations including both upstream substation connecting to the main source of power (either the grid or the in-house generator) and downstream substations for distribution of power to different load centers distributed over various parts of industries. In addition, the industry may have its own generation system to meet its energy demand and also have power management system for effectively handling power for its purpose.
Power management functionality includes load shedding, metering, measuring voltage, current, power, energy and other power-quality parameters for effective handling of power demands for various equipments and processes, power fluctuations, equipment constraints etc. It is becoming common to find this kind of setup with process industries such as refineries and power utilities. However, such setup is also applicable to other process industries such as cement, pulp and paper, petrochemical plants, fertilizers, mining and metals, water and waste water treatment plants etc.
The substations (both upstream and downstream as mentioned hereinabove) can include several power and distribution transformers, cabling, switching, reactive power and grounding equipment. These power equipment should be protected against power system anomalies like current and voltage surges and this can be accomplished by Intelligent Electronic Devices (IEDs) that provide different substation protection, control, monitoring and metering functions. The IEDs are microprocessor-based equipment that are associated with power system equipment, such as circuit breakers, generators, transformers, power lines, power cables, reactors, motors, capacitor banks etc. The IEDs can receive primary power system information like voltage and current from sensors to perform various protection and monitoring functions. Common types of IEDs include protective relaying devices, load tap changer controllers, circuit breaker controllers, recloser controllers, voltage regulators, etc. Thus a single IED can perform several protection, metering, monitoring and control functions concurrently.
Substation automation can form an important and complex aspect for maintenance and control of different equipments involved in different processes within the substation. Manual and automatic control command functions are also provided like closing and opening of switching equipment (circuit breakers and disconnectors), or raise/lower voltage levels in order to maintain the desired voltage levels. Multiple protocols exist for substation automation, which include many proprietary protocols with custom communication links. However, interoperation of devices from different vendors is highly desired for simplicity in implementation and use of substation automation devices.
The IEC61850 standard from International Electrotechnical Commission (IEC) advocates interoperability amongst Intelligent Electronic Devices (IEDs) from various manufacturers using common engineering models (for example, IEC61850 Common Engineering Model using Logical Nodes), data formats and communication protocol. Recent IEDs are therefore designed to support the IEC61850 standard for substation automation, which provides interoperability and advanced communications capabilities like GOOSE (Generic Object Oriented Substation Event) and MMS (Manufacturing Message System) communication profiles.
The power management functionality in the process industries like load shedding i.e. cutting off the electric current on certain lines/loads when the power demand becomes greater than the power supply, is currently implemented as a centralized function in distribution power automation systems and industrial power management systems. Centralized implementation of the load shedding function in a single process controller usually at the upstream substation has several shortcomings. For example, it causes high loading due to the integration of all load shedding functions for the complete power system network in a single process controller, where load shedding needs to be deployed. One skilled in the art will know that load shedding is commonly utilized for achieving power balance in electrical systems. Under heavy load, the power balance is negative and voltage support can be required. Alternately, under light load conditions, the power balance is positive and inductive compensation can be desired.
In the currently available load shedding systems the centralized load shedding process controller communicates with every IED to get input data (for example voltage, current, etc) needed for power balance based LS from various feeders at the substations to issue shed command information (information to shed a load for example) to the feeders through respective IEDs. Feeders as used herein can be an electrical circuit that may include generator, loads, conductors in conduit or a busway run, which carries a large block of power from the service equipment to a sub-feeder panel or a branch circuit panel or to some point at which the block power is broken into smaller circuits. (Feeders are also generally referred as the medium-voltage lines used to distribute electric power from a substation to consumers or to smaller substations).
Thus the availability of the LS function can depend solely on the centralized load shedding process controller. Any outage of the centralized load shedding process controller results in LS function being unavailable for the entire substation.
The power balance (and power balance principles) as described herein is balancing of supply from the generation side and demand from the load side of power in the process plant. The power balance calculations as used herein below refer to known calculations for electrical networks and components to achieve the power balance. An electrical network as would be known to one skilled in the art is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources, and switches.
In order to overcome the limitation of single point failure as outlined above, and ensuring high availability of load shedding function, additional hardware can be employed or the load shedding function is distributed in the various IEDs. In one example, this is achieved as a frequency based function available in every feeder IED that detects a rate of fall of frequency or under-frequency condition in the power network and issue trip commands to its own feeder. However, this method is a discrete method (to power demand and supply condition) and hence the amount of loads shed is always more than needed to ensure system stability.
Hence there is a need to develop a technique that allows for an improved load shedding function, based on power management principle, distributed amongst various IEDs, in one or more of substations in the process plant.